1. Technical Field
This invention relates to cordless digitizers and, more particularly, to a device for indicating the phase of a signal transmitting from the pointing device and pointing device button status.
2. Background Art
Digitizer systems for use as input devices to computers are well known in the art. In a typical digitizer system, a pointing device is moved over the sensing (or working) surface of a tablet wherein the sensing surface of the tablet defines the boundaries of an X-Y coordinate system. Sometimes, the position of the pointing device on the sensing surface of the tablet is determined by electrostatic means. For many reasons, an electro-magnetic sensing link between the pointing device and the tablet provides superior results and is preferred.
Recently, so-called "cordless digitizers" in which there is no connecting cable between the pointing device and the tablet have become popular. In an electro-magnetic version as manufactured by the assignee of this application, a digitizer tablet employs grid conductors for each of the two coordinate directions. A pointing device is the "driven" member and emits an alternating current (AC) magnetic field from its tip at a given frequency. The magnetic field emanations induce signals into the grid wires which are then used by the tablet electronics to determine the location of the pointing device. In practice, first the grid conductors for one coordinate direction are individually selected by a selector device and the induced signal therein is sensed. Then the grid conductors for the other coordinate direction are similarly selected and sensed. The amplitude characteristics of the induced signal and its magnitude are used by the tablet electronics and logic to determine how close the pointing device is to the grid conductor being sensed. When all the grid conductors have been sampled, the location of the pointing device on the tablet's sensing surface is derived.
In an implementation, where any given grid conductor crosses the sensing surface area only once, an amplitude characteristic associated with the signal induced in the grid conductor by the pointing device positioned in the middle of a group of conductors is depicted in FIG. 1. It should be noted that the pointing device location is indicated by the null in the plot of the amplitude characteristic.
In a case where a large sensing area is needed, it is often desired that the number of grid conductors be kept at a minimum to simplify construction of the tablet and reduce the manufacturing costs. This is accomplished by having an individual conductor cross the sensing surface more than once. FIG. 2 shows an implementation of a digitizer 10 where each conductor 12 crosses the sensing surface 20 of the tablet 14 twice. This configuration results in one leg of a conductor 12 being in one half of the tablet 14 and the other leg being in the other half of the tablet 14. However, an ambiguity as to the position of the pointing device 16 arises in such a configuration. The ambiguity results because a pointing device 16 placed near one side of a first leg of a conductor 12, will induce a signal in the conductor 12 as viewed from the selector 18 having the same voltage magnitude as the signal induced if the pointing device 16 had been placed the same distance from the other leg of the conductor 12. For example, a pointing device 16 placed at location "A" in FIG. 2 would induce a signal having an identical voltage magnitude as that induced had the pointing device been placed at location "B". Therefore, the aforementioned ambiguity as to the position of the pointing device 16 must be resolved in a digitizing system employing conductors 12 which cross the working surface of the tablet 14 more than once.
The ambiguity can be resolved if the phase of the signal induced in the conductors 12 is known relative to the pointing device signal. FIG. 3 depicts the amplitude/phase characteristics that would result from a pointing device 16 placed at positions "A" and "B" of FIG. 2. As can be seen the phase of the signal induced at location "A" is opposite of the signal induced at location "B". The pointing device signal will be in phase with the induced signal at one of the locations and 180 degrees out of phase at the other location. Which location corresponds to which phase is simply a matter of convention and can be made to be either way. Accordingly, by knowing the phase of the pointing device signal, the actual location of the pointing device 16 can be differentiated from the anomalous location.
In past implementations, a wired connection between the pointing device and the tablet was used to determine the phase of the field transmitting from the pointing device. However, in a cordless pointing device system, alternate means must be employed to acquire this phase information.
According to some prior art techniques, additional pick-up conductor loops are incorporated in addition to the necessary position sensing conductors. However, these extra loops require additional conductors on the sensing grid. These extra conductors increase undesirable grid capacitance and make manufacturing the grid more difficult and costly. This technique also requires that the grid employed in a cordless pointing device system to be different from that in a corded system. Therefore, a system capable of determining the phase of the pointing device signal in a cordless digitizer without the addition of extra conductors is needed.
To allow the user of the digitizer to indicate when positional information, and the like, is being transmitted from the pointing device to the tablet, the pointing device usually has one or more manually-operated buttons associated with it. Typically, when the user wishes to transmit data, one of the buttons is depressed. The pointing device then transmits an encoded signal to the digitizer tablet. The tablet electronics senses the encoded signal and performs whatever function is associated with that code.
Many different encoding schemes have been employed to transmit the aforementioned code in a cordless digitizer. Among these are Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Amplitude Modulation (AM), and variations thereof. These encoding schemes generally modify the basic pointing device signal in some way so as to create a binary code decodable by the digitizer electronics. In the past, the encoding/decoding and the phase determination schemes have been independent processes. If these two functions are combined, the resulting cordless digitizer could be simplified considerably, thereby realizing the associated manufacturing cost savings.
Wherefore, it is an object of the present invention to combine the phase determining and signal decoding functions such that each is accomplished using the same signal transmission from the pointing device.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.